mtd: cfi_cmdset_0002: Add reboot notifier for AMD flashes

Ensure that the flash device is in a quiescent state before rebooting.
The implementation is closely modeled after the cfi_cmdset_0001 reboot
notifier, commit 963a6fb0

 .

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>